The High-power Lithium-ion

NOTE: This article has been archived. Please read our new "Types of Lithium-ion" for an updated version.

Most lithium-ion batteries for portable applications are cobalt-based. The system consists of a cobalt oxide positive electrode (cathode) and a graphite carbon in the negative electrode (anode). One of the main advantages of the cobalt-based battery is its high energy density. Long run-time makes this chemistry attractive for cell phones, laptops and cameras.

The widely used cobalt-based lithium-ion has drawbacks; it offers a relatively low discharge current. A high load would overheat the pack and its safety would be jeopardized. The safety circuit of the cobalt-based battery is typically limited to a charge and discharge rate of about 1C. This means that a 2400mAh 18650 cell can only be charged and discharged with a maximum current of 2.4A. Another downside is the increase of the internal resistance that occurs with cycling and aging. After 2-3 years of use, the pack often becomes unserviceable due to a large voltage drop under load that is caused by high internal resistance. Figure 1 illustrates the crystalline structure of cobalt oxide.

Figure 1: Cathode crystalline of lithium cobalt oxide has 'layered' structures. The lithium ions are shown bound to the cobalt oxide. During discharge, the lithium ions move from the cathode to the anode. The flow reverses on charge.

In 1996, scientists succeeded in using lithium manganese oxide as a cathode material. This substance forms a three-dimensional spinel structure that improves the ion flow between the electrodes. High ion flow lowers the internal resistance and increases loading capability. The resistance stays low with cycling, however, the battery does age and the overall service life is similar to that of cobalt. Spinel has an inherently high thermal stability and needs less safety circuitry than a cobalt system.Low internal cell resistance is the key to high rate capability. This characteristic benefits fast-charging and high-current discharging. A spinel-based lithium-ion in an 18650 cell can be discharged at 20-30A with marginal heat build-up. Short one-second load pulses of twice the specified current are permissible. Some heat build-up cannot be prevented and the cell temperature should not exceed 80°C.

Figure 2: Cathode crystalline of
lithium manganese oxide has a
'three-dimensional framework structure'.
This spinel structure, which is usually composed of diamond shapes connected into a lattice, appears after initial formation. This system provides high conductivity but lower energy density.

The spinel battery also has weaknesses. One of the most significant drawbacks is the lower capacity compared to the cobalt-based system. Spinel provides roughly 1200mAh in an 18650 package, about half that of the cobalt equivalent. In spite of this, spinel still provides an energy density that is about 50% higher than that of a nickel-based equivalent.

Figure 3: Format of 18650 cell.
The dimensionsof this commonly used cell are: 18mm in diameter and 65mm in length.

Types of lithium-ion batteries

Lithium-ion has not yet reached full maturity and the technology is continually improving. The anode in today's cells is made up of a graphite mixture and the cathode is a combination of lithium and other choice metals. It should be noted that all materials in a battery have a theoretical energy density. With lithium-ion, the anode is well optimized and little improvements can be gained in terms of design changes. The cathode, however, shows promise for further enhancements. Battery research is therefore focusing on the cathode material. Another part that has potential is the electrolyte. The electrolyte serves as a reaction medium between the anode and the cathode.

The battery industry is making incremental capacity gains of 8-10% per year. This trend is expected to continue. This, however, is a far cry from Moore's Law that specifies a doubling of transistors on a chip every 18 to 24 months. Translating this increase to a battery would mean a doubling of capacity every two years. Instead of two years, lithium-ion has doubled its energy capacity in 10 years.

Today's lithium-ion comes in many "flavours" and the differences in the composition are mostly related to the cathode material. Table 1 below summarizes the most commonly used lithium-ion on the market today. For simplicity, we summarize the chemistries into four groupings, which are Cobalt, Manganese, NCM and Phosphate.

Table 1: Reference names for Li-ion batteries.We willuse the short form when appropriate.

1 Cathode material

2 Anode material

The cobalt-based lithium-ion appeared first in 1991, introduced by Sony. This battery chemistry gained quick acceptance because of its high energy density. Possibly due to lower energy density, spinel-based lithium-ion had a slower start. When introduced in 1996, the world demanded longer runtime above anything else. With the need for high current rate on many portable devices, spinel has now moved to the frontline and is in hot demand. The requirements are so great that manufacturers producing these batteries are unable to meet the demand. This is one of the reasons why so little advertising is done to promote this product. E-One Moli Energy (Canada) is a leading manufacturer of the spinel lithium-ion in cylindrical form. They are specializing in the 18650 and 26700 cell formats. Other major players of spinel-based lithium-ion are Sanyo, Panasonic and Sony.

Sony is focusing on the nickel-cobalt manganese (NCM) version. The cathode incorporates cobalt, nickel and manganese in the crystal structure that forms a multi-metal oxide material to which lithium is added. The manufacturer offers a range of different products within this battery family, catering to users that either needs high energy density or high load capability. It should be noted that these two attributes could not be combined in one and the same package; there is a compromise between the two. Note that the NCM charges to 4.10V/cell, 100mV lower than cobalt and spinel. Charging this battery chemistry to 4.20V/cell would provide higher capacities but the cycle life would be cut short. Instead of the customary 800 cycles achieved in a laboratory environment, the cycle count would be reduced to about 300.

The newest addition to the lithium-ion family is the A123 System in which nano-phosphate materials are added in the cathode. It claims to have the highest power density in W/kg of a commercially available lithium-ion battery. The cell can be continuously discharged to 100% depth-of-discharge at 35C and can endure discharge pulses as high as 100C. The phosphate-based system has a nominal voltage of about 3.3V/cell and peak charge voltage is 3.60V. This is lower than the cobalt-based lithium-ion and the battery will require a designated charger. Valance Technology was the first to commercialize the phosphate-based lithium-ion and their cells are sold under the Saphionâ name.

In Figure 4 we compare the energy density (Wh/kg) of the three lithium-ion chemistries and place them against the traditional lead acid, nickel-cadmium, nickel-metal-hydride. One can see the incremental improvement of Manganese and Phosphate over older technologies. Cobalt offers the highest energy density but is thermally less stable and cannot deliver high load currents.

Figure 4: Energy densities of common battery chemistries.

Definition of Energy Density and Power Density

Energy Density (Wh/kg) is a measure of how much energy a battery can hold. The higher the energy density, the longer the runtime will be. Lithium-ion with cobalt cathodes offer the highest energy densities. Typical applications are cell phones, laptops and digital cameras.
Power Density (W/kg) indicates how much power a battery can deliver on demand. The focus is on power bursts, such as drilling through heavy steel, rather than runtime. Manganese and phosphate-based lithium-ion, as well as nickel-based chemistries, are among the best performers. Batteries with high power density are used for power tools, medical devices and transportation systems.

An analogy between energy and power densities can be made with a water bottle. The size of the bottle is the energy density, while the opening denotes the power density. A large bottle can carry a lot of water, while a large opening can pore it quickly. The large container with a wide mouth is the best combination.

Confusion with voltages

For the last 10 years or so, the nominal voltage of lithium-ion was known to be 3.60V/cell. This was a rather handy figure because it made up for three nickel-based batteries (1.2V/cell) connected in series. Using the higher cell voltages for lithium-ion reflects in better watt/hours readings on paper and poses a marketing advantage, however, the equipment manufacturer will continue assuming the cell to be 3.60V.
The nominal voltage of a lithium-ion battery is calculated by taking a fully charged battery of about 4.20V, fully discharging it to about 3.00V at a rate of 0.5C while measuring the average voltage.

Because of the lower internal resistance, the average voltage of a spinel system will be higher than that of the cobalt-based equivalent. Pure spinel has the lowest internal resistance and the nominal cell voltage is 3.80V. The exception again is the phosphate-based lithium-ion. This system deviates the furthest from the conventional lithium-ion system

Prolonged battery life through moderation

Batteries live longer if treated in a gentle manner. High charge voltages, excessive charge rate and extreme load conditions have a negative effect on battery life. The longevity is often a direct result of the environmental stresses applied. The following guidelines suggest ways to prolong battery life.

-The time at which the battery stays at 4.20/cell should be as short as possible. Prolonged high voltage promotes corrosion, especially at elevated temperatures. Spinel is less sensitive to high voltage.

-3.92V/cell is the best upper voltage threshold for cobalt-based lithium-ion. Charging batteries to this voltage level has been shown to double cycle life. Lithium-ion systems for defense applications make use of the lower voltage threshold. The negative is a much lower capacity.

-The charge current of Li-ion should be moderate (0.5C for cobalt-based lithium-ion). The lower charge current reduces the time in which the cell resides at 4.20V. A 0.5C charge only adds marginally to the charge time over 1C because the topping charge will be shorter. A high current charge tends to push the voltage into voltage limit prematurely.

-Do not discharge lithium-ion too deeply. Instead, charge it frequently. Lithium-ion does not have memory problems like nickel-cadmium batteries. No deep discharges are needed for conditioning.

-Do not charge lithium-ion at or below freezing temperature. Although accepting charge, an irreversible plating of metallic lithium will occur that compromises the safety of the pack.

Not only does a lithium-ion battery live longer with a slower charge rate; moderate discharge rates also help. Figure 5 shows the cycle life as a function of charge and discharge rates. Observe the improved laboratory performance on a charge and discharge rate of 1C compared to 2 and 3C.

Figure 5: Longevity of lithium-ion as a function of charge and discharge rates.Lithium-cobalt enjoys the highest energy density. Manganese and phosphate systems are terminally more stable and deliver high load currents than cobalt.

Battery experts agree that the longevity of lithium-ion is shortened by other factors than charge and discharge rates. Even though incremental improvements can be achieved with careful use, our environment and the services required are not always conducive for optimal battery life. In this respect, the battery behaves much like us humans - we cannot always live a life that caters to achieve maximum life span.

Comments

On December 21, 2010 at 12:18am

Eric wrote:

This article says “The system consists of a cobalt oxide positive electrode (cathode) and a graphite carbon in the negative electrode (anode).” Anodes, however, are Positive, and Cathodes are negative. Is this a typo?

On December 22, 2010 at 11:22am

Cadex Electronics Inc. wrote:

@Eric: Maybe there is confusion, the electrode in an electrochemical cell in which oxidation takes place, releasing electrons. During discharge the negative electrode of the cell is the anode. During charge the situation reverses and the positive electrode of the cell is the anode. In a battery the anode refers to the negative electrode.

On December 22, 2010 at 11:43am

Eric wrote:

@Isidor: Thanks very much for the clarification. Your comment makes perfect sense. Anode polarity depends on the device type, and sometimes even in which mode it operates. In a device which consumes power the anode is positive (i.e., a diode), and in a device which provides power (i.e., a battery) the anode is negative.

On December 22, 2010 at 12:53pm

Hank wrote:

I recently bought an upgraded 3500 Mah Lithium-ion battery for my Cell phone to extend its usable run time. The manufacture stats the battery will reach full potential after it has been charged 5-6 times. Is there merit to this statement or not really that significant.

Thanks

On December 24, 2010 at 5:31pm

David Piermatteo wrote:

I am a contractor and recently bought a 12v lithium-ion impact driver,it came as a surprize to me that the lithium-ion product was less expensive then the nicd which has been the cordless tool power plant for quite some time. With the advantages of lith-ion,why is it cheaper. Thanks for listening.

On December 29, 2010 at 2:58pm

Richard Marks wrote:

Specific energy is Wh/kg and energy density is Wh/L I think you are mixed up

On January 4, 2011 at 9:23pm

BWMichael wrote:

David: The reason for this is probably that the lithium impact driver you are looking at will have cheap lithium cells inside which could be dangerous. After studying lithium batteries, i would not suggest buying a lithium impact driver, especially if you are using it on a regular basis in hot conditions as lithium batteries are sensitive to heat

On April 1, 2011 at 8:47am

Gabe wrote:

If I understand correctly, using an “unprotected” 18650 Li-ion battery for a single cell, high-intensity LED flashlight is not a risk since the current drain would be well below the limit that would turn my flashlight into a small pipe bomb. Is this assumption correct?

And what is the safe current draw for an unprotected cell?

Thank you for this superlative resource.

On June 5, 2011 at 10:00am

Christian StClaire wrote:

Unfortunately, manufacturers of power tools are very vague about lithium chemistries used in their tools,
They actually confuse the whole issue even for knowledgeable people that could read between the lines such as:
1- true voltage per conventional rule ( 1.2v NiCad + NiMh 3.30v LifeP04 3.6v 3.7v share for the rest of Lithium flavors)
this decifering effort is compounded by the multicell packs that often end up with the very same voltage readings.
I personally open every single lithium battery pack I ever purchase and found so many surprises, sometime good sometime bad.
one of the good one was finding LifeP04 in a fairly inexpensive lithium drill stating 21.6 volts
what was revealed when opened:
the pack had 6 prismatic cells with factory numbers.
21.6 : 6 = 3.6volts but those cells were not !! they actually were 3.30v the factory numbers after checking online also revealed Lifep04 at 1500 mah.
this was the find of the year for me and went and buy a whole bunch of those tools.
they are now powering my electric skate board and my range is over 20 miles on a single charge not to mention the zappier response from them ( before skateboard had SLA sealed lead acid )

2- capacitance for a given form factor
3- shape of cell say prismatic for LifeP04
it looks like they are confusing us on purpose to keep consumers from knowledge and they take advantage of that ignorance out there to win by clever marketing campaigns.

my name is Christian StClaire who just discovered this awesome website today,
it would be nice if my friend Vasilli Keramidas could contribute to this site since he is one of the fathers of Lithium Polymer along with the original Frenchman Jean Marie Tarascon who fully licensed Bell Core which was led to fruitation by Vassilli in the 1990’s

On February 18, 2012 at 7:51am

Lisa Mary wrote:

This one is truly one of the most instructive stuff. The concerning features mention “The system consists of a cobalt oxide positive electrode (cathode) and a graphite carbon in the negative electrode (anode).” So informative post for me. Thanks.

On March 9, 2012 at 7:38am

Dr.S.Lakshminarayanan wrote:

1.This is for Mr.Eric: A simple definition for positive and negative electrodes.
In a power supplying device, the cathode is positive and anode is negative. In a power consuming device like electrolytic cells, the anode is positive and the cathode is negative. It can be remembered easily by the following definition. Anode is the electrode where always oxidation takes place and cathode is the electrode where reduction takes place always. This definition is applicable for both power producing cells as well as power consuming cells.
2. This is Mr Richard Marks: Theoretically energy density is Wh/liter, but by convention it is being used as Whr/kg since in battery technology, the weight is the most siginificant parameter than the volume. Similarly the power density is also referred as Watts/Kg rather than Watts/liter.

On March 23, 2012 at 2:11am

http://www.battery-sale.co.uk/sony-vgp-bps8.htm wrote:

Sony Corporation today announced that it has launched a new type of lithium ion secondary battery that combines high-power and long-life performance, using olivine-type lithium iron phosphate as the cathode material. Shipment commenced in June 2009.

On May 17, 2012 at 4:45am

ARUNA BHARATHI wrote:

my research area is nanobatteries,please suggest suitable cathode material to obtain high performance characteristics of battery.

On April 30, 2013 at 1:46am

Kristof wrote:

Just a silly question..
As an example LiFePO4
If the theoritical capacity is 170 mAh/kg = 170 Ah/kg and flate voltage of 3.7V
Then theoritacally we must reach E = CxU = 170 x 3.7 = 629 Wh/Kg
It is indicated a value of 110 Wh/kg on your figure 4. Do i´m wrong in my way to calcul it? Why there is a factor 5.7 in comparison with the expected value?

On April 30, 2013 at 1:48am

Kristof wrote:

My mistake 170 mAh/g…

On May 7, 2013 at 1:40am

Andreas wrote:

Kristof, your calculation is not wrong, however, a cell is made up out of more than just the electrode materials. You’ll also have to consider, electrolyte, casing et cetera. Additionally, theoretical capacity assumes 100% utilization of the active materials, which unfortunately ‘never’ happens.

Hope this helped.

On May 7, 2013 at 2:35am

Kristof wrote:

Many thanks Andreas for taking the time to give an answer.
I see the idea, then your are represented the máximum enable to be reach by technology and reported in the SOA. I was curious to know if it was posible to estimate the real characteristics from the theoritical one + the configuration of the cell + electrochemical test as a starting point.
Then the numbers presented will change significantly if we are talking about coin cell, cylindrical cell, pouch cell… It´s difficult to compare different materials with different “packaging” and different rate of charge, ect…
Typically, it is difficult to plot in the same graph a lead acid battery in a pack cell of 6 single cells charged at C/5 and a LiFePO4 in CR2032 format and charged at 10C. With two total weights very different. For example, why to not replace all the lead acid battery onboards cars by a lighter li-ion pack? It is not only a question of safety or price, but also because lead acid technology is using for energy application and lithium ion for both power and energy.
Thanks agains, and if you have some reference about my philosophic interrogation to prevent the charasteristic of a system then i will appreciate.
Have a nice day.

On June 3, 2013 at 9:20am

bruce brown wrote:

can you respond to this confusion between electrode (charge holding) density often quoted in mAH/g as opposed to th battery energy density often quoted in Whr/g i there a simple way to convert a breakthrough in electrode charge density into battery energy storage density eg a 1810mAh/g electrode in a GEO/GE/C composite is around 6 times that of standard cobolt li ion carbon electrode but what does that do to the energy density of the cell that it resides in duble triple or just alter the charge discharge rate. which is the chicken and which is the egg with all these new battery and electrode breakthorughs

On March 26, 2014 at 6:42am

PETER wrote:

I would like to know the impact of LiMn204 conductivity as cathode material on the performance of lithium ion batteries,some one who has details information about this issue can help me.thx

On April 14, 2014 at 4:16pm

Bill Bower wrote:

As an end consumer.. I avoid smart battieries like I would AIDs. I have a number of expensive useless devices that have smart batteries. Smart meaning instead of discarding expired batteries you discard the device.. because it’s not supported.. I want to know how to bypass the circuitry to use conventional battery packs when the replacements are not available? Any takers? In the meantime… I do my damnedest to avoid purchasing devices with smart batteries.. It’s not a selling point. I’m not the only one that feels this way either, don’t kid yourself.

On November 2, 2014 at 11:57am

Walker wrote:

@Bill Bower: From your post, it seems you’re interested in keeping your electronics alive for as long as possible for any number of reasons. I don’t blame you, if something still has a use, why throw it away simply because the battery’s dead?!

The answer is simple: get comfortable rebuilding your own battery packs. Easier said than done though. What we’re running into is that battery chemistries are not all created equal. Someone’s Lithium Cobalt cell wont be the same as someone else’s, and heaven forbid you find a LiFePO4 (Iron Phosphate) cell and put it into something meant for LCO! That would probably be the end of that particular device, hopefully while you’re around to watch it so it doesn’t catch the room on fire.

If you want to use standard lead acid, Alkaline, or NiMH batteries in a newer lithium powered device, there’s a lot of engineering that needs to happen (backyard engineering works sometimes…) before all the pieces fall into place: Voltage levels, charging considerations, capacity, weight, safety, safety, safety.

Batteries in today’s electronics do one of two things: last as long as the “useful life” of the product they’re permanently installed in, or are made to be replaceable to work with those devices that have a “useful life” longer than the battery. When the company goes out of business or stops selling batteries for that particular product, the aftermarket pops up and usually takes over with the newest tech. (BatteriesPlus.com is your friend!)

On December 30, 2014 at 4:07am

Shiju wrote:

If we charge Li-ion battery with low current capacity charging circuit so that it will be charged with 0.1C during CC charging will it improve life? In my application 99 % of the life of the instrument it will work from the wall cube power supply. And the cell will be function as backup

On December 31, 2014 at 2:57pm

Walker wrote:

@Shiju: Low charge rates will typically lengthen the battery life on a sliding scale. The more slowly you charge or discharge a lithium battery, the less you’ll “damage” it and reduce the capacity of that battery.

That being said, if you’re using a rechargeable lithium battery as a backup power source, make sure you understand the lifespan considerations of the device since most LCO lithium variants have a 3-4 year shelf life before they’re useless - regardless of cycles. Also, a rechargeable lithium battery probably shouldn’t “float” at its highest voltage level while ready to provide backup power. If you let the battery “float” at something closer to 80% SoC (instead of 95-100%), you’ll see a much higher standby life of that battery.

I guess what I’m saying is that in your application, the “shelf life” will be a much larger issue than cycle life and you should work on optimizing that aspect of the application - even consider a different battery chemistry like NiMH or replaceable non-rechargeable lithium to lengthen useful calendar life for the application.

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